Electric liquid dispensing guns are designed to rapidly discharge droplets or strands of material onto a moving substrate, such as woven or non-woven fabrics, paper or other substrate materials. Dispensing guns of this type include a liquid passage that communicates between a pressurized liquid supply and a valve mechanism provided at the end of the liquid passage. The valve mechanism is typically a moveable plunger positioned to selectively obstruct a dispensing orifice formed in a valve seat. The plunger is extended and retracted relative to the valve seat in a controlled manner by a solenoid for providing repeatable and accurate dispense patterns of liquid material onto the substrate. It is important in the operation of the dispensing gun that the solenoid acts upon the plunger to quickly open and close the orifice when desired.
Dispensing systems have been developed that employ driver circuits to control operation of the solenoid within the dispensing gun. To open the valve, the driver circuit applies a fast pull-in current to the solenoid coil to quickly retract the plunger and open the dispensing orifice at the beginning of a dispense cycle. The driver circuit maintains a minimal holding current which holds the plunger in an open position while minimizing the amount of heat build-up in the solenoid coil during the dispense cycle. Finally, the driver circuit provides a fast demagnetization of the solenoid so the plunger is quickly closed upon the orifice at the end of the dispense cycle.
Closing of the plunger is generally achieved by a spring mechanism connected to one end of the plunger. When the solenoid is sufficiently demagnetized, the stored energy in the compressed spring mechanism forces the plunger to the closed position and in sealing engagement with the dispensing orifice. One example of such a dispensing system is set forth in U.S. Pat. No. 5,812,355, owned by the assignee of the present invention, the disclosure of which is incorporated herein by reference in its entirety.
Known electric dispensing guns and driver circuits have several drawbacks. In particular, current electric dispensing guns must typically quench the magnetic field in the solenoid before the plunger is forced to the closed position by the spring mechanism. The quenching time is dependent on the amount of energy stored in the solenoid's magnetic circuit and the voltage applied to the solenoid. All throughout the quenching process, the plunger is stuck in the retracted or open position until the solenoid is sufficiently demagnetized for the mechanical spring force to reposition the plunger to the closed position. The required quenching time of the solenoid's magnetic circuit reduces how quickly the orifice can be opened and closed, and thus, significantly affects the dispensing pattern that may be generated by the dispensing gun.
Accordingly, there is a need for an improved electric dispensing gun and driver circuit that reduces the time required to close an electric dispensing gun.